The invention relates to a device and to a process for the remelting of glass semifinished products. It is a matter here of glass semifinished products that are present in the form of bars. The bars have mostly a circular cross section. The remelting of such glass semifinished products is necessary for the further processing. It is associated mostly with a portioning of glass.
In principle glassxe2x80x94either pre-portioned or in relatively large quantitiesxe2x80x94can be melted with the necessity of a subsequent portioning.
The process of the remelting of portioned glass is problematical. From a certain temperature onward, namely, there occurs an adhesion of the glass to the underlayer or, respectively, to the forming tool. The so-called adhesion temperature (1010 dPas) lies below the softening temperature (107.6 dPas), so that in the remelting there occurs an adhesion of the glass to the wall of the melting vessel.
There could be considered cooling the wall, so that in the wall zone the adhesion temperature is not reached. But in the heating-up process the cooling cannot be strong enough for the contact temperature to remain below the adhesion temperature. Furthermore, there occur excessively great inhomogeneities of the temperature.
As remedy, work is being done with separating agents of various type. These lead, however, to undesired alterations of the glass surface.
The other possibility lies then in melting larger quantities and then portioning them. There the glass semifinished product is usually prepared not in the form of regular bodies, but it can be melted-up in the crucible as glass fragments. In the first place, however, this process is in general a discontinuous one; continuous processes require special measures and therewith special expenditures. In the second place, here in all cases hollow-space forms, which are filled with air or another gas. To the glass fragments adsorbed gas adheres. When the fragment surfaces pass into the melt, then the adsorbed gas is released from the fragment surfaces, and bubbles form. In order to remove these bubbles again from the melt, a strong increase of the melt temperature to refining temperature is necessary. This means that the melting processes must be run through once again. This is highly unfavorable and uneconomical energy-wise.
It is also possible, when using glass bars, to heat up the free bar end and therewith to do the melting. There, to be sure, the above-mentioned problems do not arise. Here, however, there occurs a continuous run-off of the glass in the form of a downward-tapering glass stream. A drop-portioning is not possible.
JP 63/236729A shows a device with which a glass fiber is drawn. At the outlet of a revolving nozzle there arises a xe2x80x9cdraw onionxe2x80x9d (Ziehzwiebel). This device, therefore, does not serve for the remelting and portioning of glass bars.
Underlying the invention is the problem of providing steps with which a remelting and portioning of glass semifinished material is possible without there arising the problems of adhesion, of alteration of the glass surface, or of the gas inclusion; further, an energy-wise and economically favorable process is to be given, with which semifinished material can be rewarmed and made available for the shaping.
This problem is solved by the independent claims. The solution is very simple and requires a relatively slight expenditure.
One proceeds, accordingly, from a receiving shell which has an upper receiving opening for the introduction of a glass bar, and a lower outlet opening. To the shell there is assigned a crucible. The crucible is open at the top, so that the shell can plunge, at least in a part of its length, into the space enclosed by the crucible wall. The crucible has a runoff in its lower region. There, any type of crucible heating is possible, for example a resistance heating and a radiation heating. An advantageous form of execution is the providing of a coil which surrounds the wall of the crucible in a known manner. The coil serves for the coupling of electric energy into the crucible contents.
The shell is suitably designed and dimensioned in such manner that its inner contour corresponds to the outer contour of the glass bars to be processed. The shell does not absolutely have to be vertical. It can also be slanted with respect to the vertical. Furthermore, instead of one shell there can also be provided several shells.
By corresponding allocation of shell and crucible it is achieved that the glass bar in the shell melts at its lower end. In the crucible itself there was previously melted up a glass bath. The glass bar melts off slowly at its lower end. The melting-off and flowing-off material passes continuously into the bath mentioned, present in the crucible.
When energy costs play no role, then the semi-finished products can simply be melted up again below, refined at low viscosities and then fed to the shaping operation. This heating to melting or refining temperature can be eliminated by the invention. The advantage becomes clear if one considers that the refining temperature can lie at approximately 1600xc2x0 C., for the shaping, however, only temperatures on the order of 1100xc2x0 C. are necessary.
What is essential for the device in its general form is merely that the receiving shell be arranged over a container that contains a glass melt. The glass melt is portionedxe2x80x94possibly only after further transport. Also an intermediately engaged homogenization step is conceivable. This becomes necessary if the introduced glass is a glass other than that which is present in the container.
An essential advantage of the process of the invention for the remelting and portioning is the continuity through the steady following-up of the glass bars. There the shape of the glass bar ends to be added to one another is decisive in respect to the avoiding or minimizing of bubble inclusions.
Untreated bar ends, by reason of shrinkage in the production, have a concave curvature. The setting on one another of two such ends would lead to a gas inclusion in the cavity arising between the ends, which would later become visible in the feeder as bubble thrust, therefore spoilage.
A bar end is easily influenceable by the casting mold used for the production. The shaping of the (other) bar end must occur by mechanical reworking and is expensive. But at least it is possible, if necessary, to free it from the concave curvature in this way. As suitable pairs of bar ends there have proved the combinations in which the gas to be expelled can escape and is not trapped.
If the same amount of melt that is removed in a portioned manner is added to the crucible, then the volume of the melt in the crucible remains constant. The level of the melt, therefore, likewise remains constant. If these conditions are maintained, then no level re-setting of the receiving shell is necessary.
For reasons of energy saving the volume of the crucible should be just as great as that of the required portions being processed. Any increase of this volume is unfavorable in respect to the energy saving of the process.